Pointing devices for entering commands into a computer, such as mouses, joy sticks, x-y tablets, wire pens and track balls, are well known in the art. Some pointing devices include a rotatable ball and one or more depressible buttons. Electronic encoders sense rotation of the ball and generate a signal indicative of the ball's rotation to control movement of a cursor on the screen of the computer. A joy stick-type pointing device moves a cursor based on the position of the joy stick. Other pointing devices similarly enter commands into a computer. Depressing the button, whether on a mouse, joy stick or track ball, permits the user to enter various commands into the computer, based on the location of the cursor. For example, depressing the button may pull down a menu; create starting, ending, or other points in a graphic pattern on the screen; move objects to different locations on the screen; and the like.
Pointing devices having a rotatable ball are generally grouped into two categories, a mouse type or a track ball type. In a mouse-type device, the ball extends from the bottom of a housing for contacting a table top or other work surface. Movement of the housing across the table top causes rotation of the ball and the appropriate movements of the cursor on the screen. FIG. 1 of U.S. Pat. No. 4,612,539, to Hosogoe et al., illustrates a prior art mouse-type device. In a track ball-type pointing device, the rotatable ball faces upward in a housing positioned on a table top. The rotatable ball is exposed above the housing for rotation by the hand of a user. The housing may include one or more depressible buttons to enter commands into the computer, based on the position of the cursor on the screen. U.S. Pat. No. 4,786,892, to Kubo et al., is an example of a pointing device which is alternatively usable as a mouse or track ball-type device.
Prior art pointing devices, particularly a mouse or a track ball type, are not easily usable with laptop or portable computers. Pointing devices having a ball often require a work surface against which to roll the ball or upon which the housing of the track ball must sit. A laptop computer is often used in an environment where a work surface is not provided. For example, laptop or portable computers may be used on an airplane or while sitting in a chair with no desk or in a chair spaced from a desk. Other computers, such as portable computers and the standard personal computer, may be used in an environment which does not provide a work surface to facilitate operation of the pointing device.